1. Field of the Invention
The present invention generally relates to an automotive antiskid control system. More particularly, the present invention relates to an automotive antiskid control system which can adjust the brake pressure of each wheel so that the slip state of each wheel can match with the target slip state of each wheel.
2. Related Art
An antiskid control system which can shorten the stopping distance by maintaining the slip state of each wheel to the target slip state while braking has been known. That is, the adjustment of brake pressure is started on condition that the slip state of each wheel has exceeded the target slip state (slip ratio: approximately 20%) and thereby maintaining the slip state of each wheel to the target slip state. Normally, this system is constructed so that the brake pressure corresponding to an amount of brake pedal depression is continuously transmitted as it is to the wheel cylinder of each wheel from the start of braking until the slip state of each wheel exceeds the target slip state.
However, if the vehicle is braked under a condition that load allocation to each wheel is not even, such as if the vehicle is turning, only the driving wheels have snow chains wound therearound or the arrangement of passengers is lopsided, the smaller the load allocated to a wheel is, the earlier this wheel is locked and the earlier the timing of this wheel exceeding the target slip ratio is. For this reason, in the above case, the antiskid control start timing varies from wheel to wheel and the driver has a sense of incongruity, which poses a problem.
Also in such a case where the vehicle is braked while running on a split road or on a road surface like a waste land on which the left wheels and the right wheels have different coefficients of friction on the grounding surfaces, the smaller the coefficient of friction of the road surface is on which a wheel grounds, the earlier the antiskid control starts with respect to this wheel. Therefore, the driver has a sense of incongruity due to the unbalanced control start timing, which poses another problem.
Also when the friction state of each wheel or the air pressure thereof is unbalanced, a difference among the locking tendency of each wheel occurs, which also poses a similar problem.
To counter a problem occurred when the vehicle is braked while turning, in the case where the lateral acceleration when the vehicle is braked while turning becomes equal to or larger than the specified value before the antiskid control starts, such techniques have been proposed as (1) restraining the brake pressure increases of the rear wheels (Japanese Unexamined Patent Publication No. 61-1564), "(2) holding or reducing the brake pressures of the inside turning wheels (Japanese Unexamined Patent Publication No. Hei. 1-178060), and (3) increasing the brake pressures of the inside turning wheels at lower increasing rates than those of the brake pressures of the outside turning wheels (Japanese Unexamined Patent Publication No. Hei. 1-215657). However, these techniques have never been so advanced as to consider the load allocation to each wheel or ease the sense of incongruity of the driver as to the antiskid control start timing as described above.
On the other hand, the applicants for this patent have proposed a technique, as stated in the Japanese Unexamined Patent Publication No. Hei. 7-101327, which is to prevent the unbalanced timing of the antiskid control start among the four wheels when the vehicle is turning. This technique has not yet been publicized as of japanese application date of the present invention, being not a publicized prior art. The purpose of this technique is "for such a system that executes the antiskid control with the antiskid determination value V.sub.S =0.8V.sub.O with respect to the body speed V.sub.O to slightly restrain the brake pressure increase rate of a wheel when the wheel speed of the wheel falls below the first determination value V.sub.D (V.sub.O &gt;V.sub.D &gt;V.sub.S) and furthermore to change the pressure increase rate to be more restrained when the wheel speed falls below the second judgment value V.sub.B (V.sub.O &gt;V.sub.D &gt;V.sub.B &gt;V.sub.S)". By using this technique, the start timings of the antiskid control for all four wheels can almost be coincided with each other.
However, in this proposed technique, it is conceivable that the brake pressure increases of all the four wheels are restrained. For example, upon the start of braking, all the four wheels may start slipping in the same way. In this case, as the brake pressure increases of all the four wheels are restrained, it is conceivable that the antiskid control is started before the driver obtains the feeling of deceleration as he expected. That is, in the previous proposal of the applicant, it is probable that the sense of incongruity of the driver that the antiskid control start timing is too early can be eliminated and nevertheless a new feeling of congruity that he can not obtain a feeling of deceleration may occur to the driver.